Detailed in situ observations of the olivine-spinel phase transformation in Fe2SiO4 were performed using a cubic anvil type of high-pressure and high-temperature X ray diffraction apparatus combined with synchrotron radiation. An accurate equilibrium phase boundary, density increase at the transition, and thermal expansion of spinel phase were determined as follows: Transition pressure P (GPa)=2.75+2.5¿10-3 T (¿C); Δ&rgr; at 5.3 GPa and 1000¿C=9.5%; mean volume expansion between 25¿C and 1000¿C at 5.3 GPa=2.0¿10-5 K-1. Comparisons of the observed and calculated equilibrium phase boundaries indicate that when the effect of cation disorder is taken into account, the observed phase boundary becomes consistent with thermochemical data. The thermal expansion of the spinel phase observed in its stable field differs significantly from that observed at atmospheric pressure. Nonuniform fomation of the diffraction peaks of the spinel phase was not observed in the present study, which indicates that the mechanism of the olivine-spinel transition may depend on the stress state of the specimen. ¿American Geophysical Union 1987